1. Field of the Invention
The present invention relates to a method of sealing a semiconductor element that is mounted on a gold-plated printed circuit board.
2. Description of the Prior Art
Examples of conventional optical semiconductors include light-receiving elements such as the photodiodes of various optical sensors and phototransistors, as well as light-emitting diodes (LED). One known method of molding such optical semiconductors involves, for example, mounting the semiconductor element on a gold-plated printed circuit board, and following wire-bonding, sealing the semiconductor element using a molding technique such as potting or transfer molding.
Epoxy resins have conventionally been used as the sealing materials, but improvements in the performance levels of elements, and the resulting increases in optical energy and heat generated by the elements, have lead to discoloration and degradation of the epoxy resins becoming a significant issue (see non-patent reference 1). Silicone resins are possible alternative resins that offer excellent durability, but because silicone resins exhibit poor adhesion when employed within the method described above, simple replacement of epoxy resins with silicone resins is problematic (see non-patent reference 2).
[Non-patent Reference 1]
Electronic Materials, September Issue (No. 44(9)), pp. 86 to 89 (published by Kogyo Chosakai Publishing, Inc.)
[Non-patent Reference 2]
Electronic Materials, September Issue (No. 44(9)), pp. 90 to 93 (published by Kogyo Chosakai Publishing, Inc.)
On the other hand, known methods of bonding together inorganic substances and silicone materials include methods in which the inorganic substance is first subjected to preliminary surface treatment with a silane coupling agent, and methods in which a silane coupling agent is added to the silicone material. Specific examples include methods in which a silane coupling agent is sprayed onto the inorganic substance, either as a neat liquid or in the form of a solution that has been diluted with a solvent (either an organic solvent or water), and methods in which the inorganic substance is immersed in the silane coupling agent and then dried. However, when the inorganic substance is gold (such as gold plating), achieving a favorable degree of adhesion has proven difficult.